How can I add a scale reference to the Z axis
Easiest option

Orthographic views tend to be useful for this. Even if you're eyeballing there will be less distortion in the perception of location. Orthographic views will show a grid in all three orientations. -- font, left, top ... and their reverse: back, right, bottom. See the Blender manual for all options, and menus relating to this



Here Front, Top and Left are visible. 

possibly interesting option

An addon called 'Chromoly Ruler' or just 'Ruler', is included in the addons_contrib. If you get blender from somewhere like builder.blender.org. It looks like this